How long does it take to get in shape?

First, the good news. Your body actually starts getting stronger and healthier just hours after you start working out. But if you’re wondering how long it will take to rock a six-pack—well, you’ll have to be a bit more patient. A few years ago, exercise scientist Megan Anderson and her colleagues at the University of Wisconsin-La Crosse put 25 sedentary volunteers through an intense six-week exercise program modeled on the bold claims made by companies such as Bowflex and Body-for-Life. Despite sticking to the program religiously, zero percent of the subjects developed instant washboard abs. In fact, a panel of six judges could detect no differences whatsoever in their physical attractiveness before and after the program.

That doesn’t mean nothing was happening. After just a few strength training sessions, your brain learns to recruit more muscle fibers and make them contract all at once to produce a greater force.

This “neural activation” kicks in after only a few workouts, allowing you to get stronger almost immediately, well before your muscles get noticeably bigger. Further strength gains come as the individual muscle fibers within your muscles get bigger, which starts in as little as two weeks if you’re training intensely. But it takes longer for these changes to be noticeable: even with sophisticated lab equipment, researchers can’t usually detect changes in fat and muscle composition until after about nine weeks of training.

Similarly, a University of Tokyo study published in 2010 saw the biggest increases in strength after two months and the biggest boost in muscle size after three months. To achieve these rapid gains, the subjects were doing four very hard workouts a week. For the average person at the gym, it will take six months or more to see significant sculpting of the body—even though strength has been increasing from day one. Weight loss is more difficult to predict, because it depends on your starting point, your health history, your genetics, and your diet as well as your workout routine.

But like strength training, aerobic exercise produces major health and performance benefits long before you see them in the mirror. Aerobic exercise increases the number of mitochondria, which are essentially the “cellular power plants” in your muscles that use oxygen to produce energy: the more mitochondria you have, the farther and faster you can run, and the more fat your muscles will burn. Studies have found that about six weeks of training will boost mitochondria levels by 50 to 100 percent. Health benefits, on the other hand, kick in after a single bout of aerobic exercise. For about 48 hours after a workout, your muscles will be consuming more glucose than usual, helping to bring down blood-sugar levels. After a few workouts, your insulin sensitivity will begin to improve, offering further control of blood sugar. The bottom line: “Getting in shape” is a journey that extends over months and even years, but the process—and the benefits—start as soon as you begin exercising.

So if you’re having trouble staying motivated without immediate physical changes, track your strength and endurance gains along the way.

Am I exercising enough?

This is a hugely controversial question, but not in the way you might expect. Almost all experts agree about what the science says—but they’re bitterly divided about what message they should convey to the public. Decades of research have made two things crystal clear:

1. Every bit of exercise helps, even in scraps as short as 10 minutes.

2. More is almost always better.

The challenge is conveying the second message without discouraging the people who are still struggling with the first. And there are a lot of people struggling: in 2008, the Centers for Disease Control and Prevention found that a quarter of Americans hadn’t done any physical activity at all in the previous month. And less than half met the modest government-mandated Healthy People 2010 goal of 30 minutes of moderate exercise five times a week. In Canada, only a third of people are meeting a similar goal of 30 minutes at a moderate effort, four times a week.

From a public health perspective, the top priority is getting those inactive people to start moving, even a little bit. Going from zero to slightly active offers the biggest possible health boost, according to a recent National Institutes of Health study that followed 250,000 men and women between the ages of 50 and 71. Those who were just slightly active but didn’t manage to meet the exercise guidelines were 30 percent less likely to die than those who were totally inactive. Stepping it up to moderate exercise reduced risk by only eight more percent, and adding in some vigorous exercise subtracted an additional 12 percent. When you combine that data, you find that getting half an hour of moderate to vigorous exercise five times a week cuts your risk of dying from all causes in half. So far, so good. The controversy starts when you ask what happens if you do more exercise than the government guidelines recommend.

According to researchers like Paul Williams of the Lawrence Berkeley National Laboratory, you keep piling up more and more benefits. Williams has been following a cohort of 120,000 runners since 1991, tracking how much they run and what happens to their health. With this enormous sample size, his National Runners’ Health Study has been able to uncover a pronounced “dose–response” relationship between aerobic exercise and health: the more you do, and the harder you do it, the more benefits you get.

What kind of benefits, you ask? Well, in a series of studies stretching back over a decade, Williams has found that the risk of everything from big killers like diabetes, stroke, and heart attack to less common conditions like glaucoma, cataracts, and macular degeneration can be reduced by as much as 70 percent by going beyond the standard exercise guidelines. In every case, the benefits relate to both how much running the subjects do and how fast they do it. For example, every additional mile in your average daily run lowers your glaucoma risk by 8 percent.

And speeding up your 10K time by one meter per second (the difference between, say, finishing times of 53 minutes and 40 minutes) lowers heart attack risk by about 50 percent. It is true that overdoing your workout regimen can weaken your immune system, especially if you’re not eating and sleeping well. And elite athletes training for hours a day at extremely high intensities sometime suffer from “overtraining” syndrome. But the message of Williams’s research is that the risks of doing too much—and these results apply to all forms of aerobic exercise, not just running—pale in comparison to the benefits of doing a bit more.

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